A study was conducted to assess the effectiveness of trivalent influenza vaccine in preventing influenza and reducing absenteeism in health-care workers of Gifu Red Cross Hospital during the 2002-2003 influenza season. Questionnaires were distributed to 370 health-care workers, and 366 were returned showing that 237 had received influenza vaccine and 129 were unvaccinated. The criterion for influenza was febrile episodes with a positive reaction on a rapid antigen detection test. Among vaccine recipients, the number of influenza infections was 3.4 per 100 subjects, compared with 8.5 per 100 subjects in nonrecipients (P = 0.034), and the number of days absent from work per 100 subjects was 9.5, compared with 15.1 per 100 subjects in nonrecipients (P = 0.0003). The prevalence of adverse reactions to the vaccine was rather low. No serious adverse reactions were recorded and no one was absent from work because of adverse reactions. It is concluded that influenza vaccine is effective in preventing influenza and reducing absenteeism in health-care workers. The results of this study support recommendations for influenza vaccination in health-care workers.